Each time a eukaryotic cell divides it must duplicate, not only its genomic DNA, but the underlying chromatin structure, as well. The faithful propagation of chromatin structure is necessary for the packaging and protection of the eukaryotic genome as well as for the maintenance of epigenetically inherited transcriptional programs. The primary goal of our research program is to decipher the mechanisms by which the primary protein components of chromatin (the core histones H2A, H2B H3 and H3 and the linker histone H1) are brought together with genomic DNA to form chromatin. The specific focus of this proposal is the characterization of a class of enzymes known as type B histone acetyltransferases. These enzymes are responsible for the post-translational acetylation of newly synthesized histones. While it has been known for decades that newly synthesized histones are acetylated during the process of chromatin assembly, the function of these modifications is not known. We have proposed a number of studies that will help to identify the role of the type B histone acetyltransferase Hat1p in chromatin assembly. The first specific aim utilizes S. cerevisiae as a model system to study Hat1p and the acetylation of newly synthesized histones. We will use experimental systems that will allow us to directly assay for the effect of Hat1p on chromatin assembly in several different contexts. In addition, will use a variety of yeast molecular genetic techniques to decipher the unique and overlapping functions of the multiple sites of acetylation that have been identified on newly synthesized histones. Finally, we will continue the isolation and characterization of a novel chromatin assembly factor that we have identified in yeast extracts. The second specific aim extends our studies of the yeast enzyme, to the characterization of mammalian Hat1. We will use biochemical techniques to isolate and characterize complexes containing the human Hat1 enzyme. In addition, we will characterize a Hat1 mouse knockout model in order to identify the function of this type B histone acetyltransferase in a complex organism.